As one fundamental method to strengthen steel materials, methods of utilizing phase transformation by heat treatment, particularly martensitic transformation, have widely been practiced. Since a steel pipe made of medium carbon steel or high carbon steel (typically, a steel pipe of low alloy steel or medium alloy steel) exhibits excellent strength and toughness after being quenched and tempered, methods for strengthening steel materials by quenching and tempering have been used in many applications including machine structural members, and steel products for oil well use. The strength of steel can be remarkably increased by quenching, and this strengthening effect depends on C content in the steel. However, since martensite structure as quenched is generally brittle, it is subjected to tempering at a temperature not more than Ac1 transformation point after quenching, thereby improving its toughness.
To obtain a martensite structure by quenching low alloy steel or medium alloy steel, rapid cooling such as water quenching is necessary. If cooling rate is insufficient, a structure softer than martensite, such as bainite, would be mixed with martensite so that sufficient quenching effect cannot be achieved.
In quenching treatment of steel materials, quench cracking may become an issue. As described above, when a steel product is rapidly cooled, it is inevitably impossible to uniformly cool the entire steel product, and then thermal stress is generated in the steel product, attributable to the difference in the contraction rate between an early cooled portion and a late cooled portion. Further, when a quenching treatment causes martensitic transformation, transformation stress is generated as a result of occurrence of volume expansion due to transformation from austenite to martensite. The volume expansion depends on a C content in steel, and the more the C content is, the larger the volume expansion becomes. Therefore, the steel having a high C content is prone to have large transformation stress in a quenching stage, and is highly likely to cause quench cracking.
In particular, when the steel product to be quenched has a tubular shape, it exhibits a very complex stress state, compared to other shapes such as flat plate shape, or a bar/wire shape. For this reason, if a tubular steel product having a high C content is subjected to rapid cooling, such as water quenching, crack susceptibility remarkably increases and quench cracking frequently occurs, resulting in a very poor yield of the product.
Therefore, when a steel pipe containing a high carbon among low alloy steels and medium alloy steels is quenched, the cooling rate during the quenching treatment is controlled by performing oil quenching which has a lower cooling capacity compared to water quenching, or performing relatively slow cooling by mist cooling, in order to prevent quench cracking and increase the yield of product.
However, when such quenching means is adopted, a sufficient amount of martensite structure cannot be obtained, resulting in a mixed microstructure including a considerable amount of bainite which occurs at a comparatively elevated temperature. For that reason, there arises a problem that even if quenching and tempering is applied, it is not possible to fully make use of excellent toughness of tempered martensite structure, thereby resulting in deterioration of high toughness of a product steel pipe.
While martensite structure is capitalized in a steel pipe of low alloy steel or medium alloy steel as described above, a martensitic stainless steel pipe, which can easily achieve high strength, is widely used in the field of a stainless steel pipe as well for various applications for which strength and corrosion resistance are required. Particularly in recent years, from energy-related circumstances, martensitic stainless steel pipes are extensively used as oil well country goods for collecting oil and natural gas.
That is, the environment of wells (oil wells) for collecting oil and natural gas has become more and more hostile in recent years, and in addition to the increase of pressure associated with the increase of drilling depth, the number of wells which contain significant amounts of corrosive components such as wet carbon dioxide gas, hydrogen sulfide, and chlorine ions have been increasing. Accordingly, while the increase of the strength of material is demanded, corrosion of the material due to corrosive components as described above and embrittlement caused thereby have become an issue, and thus there is a growing demand for oil well pipes having excellent corrosion resistance.
Under such circumstances, martensitic stainless steels are widely used in environments containing wet carbon dioxide gas of relatively low temperature, since the martensitic stainless steel has excellent resistance to carbon dioxide gas corrosion although it may not have sufficient resistance to sulfide stress corrosion cracking caused by hydrogen sulfide. Typical examples thereof include an oil well pipe of 13Cr type steel (having a Cr content of 12 to 14%) of L80 grade specified by API (American Petroleum Institute).
Generally, it is common to apply quenching and tempering treatments for the martensitic stainless steel, and the 13Cr steel of API L80 grade is no exception. However, since the 13Cr steel has a martensitic transformation starting temperature (Ms point) of about 300° C., which is lower than that of low alloy steel, and has a large hardenability, it exhibits high susceptibility to quench cracking.
Particularly, when a tubular steel product is quenched, it exhibits a very complex stress state, compared with the case of a sheet/plate or bar material, and when it is subjected to water cooling, quench cracking occurs; therefore, it is necessary to adopt a process with a slow cooling rate such as cooling in air (natural air cooling), forced air cooling, and slow mist cooling. For this reason, in the production of the 13Cr-type oil well pipe of L80 grade, air quenching is performed to prevent quench cracking. Since this type of alloy steel has a large hardenability, martensitization can be achieved even when the cooling rate at the time of the quenching treatment is slow.
However, although this method can be effective in preventing quench cracking, problems arise such that the productivity is low since the cooling rate is slow, and besides, various properties including the resistance to sulfide stress-corrosion cracking deteriorate.
In this way, even in a steel pipe of low alloy steel or medium alloy steel, or further in a martensitic stainless steel pipe, there is a problem of quench cracking in a quenching treatment, and thus there is a greater need for solving this problem particularly in a steel pipe, compared with a sheet/plate material and a bar material.
Conventionally, there have been proposed several techniques to solve such a quench cracking problem. For example, Patent Literature 1 discloses, as a method for preventing quench cracking of a steel pipe containing 0.2 to 1.2% of C, a method for quenching a steel pipe made of a medium or high carbon type of steel, in which cooling in a quenching process is performed only from an inner surface of the steel pipe, and whenever necessary, the steel pipe is rotated during cooling.
In the literature, it is suggested that: when the outer surface of the steel pipe is rapidly cooled, martensitic transformation of the outer surface precedes, and the brittle martensite structure of the outer surface cannot withstand the transformation stress due to a delayed martensitic transformation of the inner surface, thus leading to quench cracking; and it is possible to appropriately countervail the transformation stress and the thermal stress by cooling the steel pipe from the inner surface. However, there is a problem that performing the cooling of the inner surface of a steel pipe involves technical difficulties compared with the cooling of the outer surface.
Patent Literature 2 discloses, as a method for producing a steel pipe having a microstructure principally composed of martensite by applying quenching and tempering treatments for a Cr-based stainless steel pipe containing 0.1 to 0.3% of C and 11.0 to 15.0% of Cr, a method for producing a martensitic stainless steel pipe in which the steel pipe is quenched at an average cooling rate of not less than 8° C./sec in a temperature range from Ms point to Mf point (temperature at which martensitic transformation ends) when performing the quenching treatment, and thereafter the steel pipe is subjected to the tempering treatment. By ensuring the above-described cooling rate, it is possible to prevent the formation of retained austenite, thereby obtaining a microstructure principally composed of martensite.
However, in order to prevent quench cracking even in rapid cooling such as water quenching, the production method of Patent Literature 2 requires that cooling be performed only from the inner surface of a steel pipe, and further, as needed, the steel pipe be rotated, so that a problem similar to that of the quenching method according to Patent Literature 1 arises when put into commercial use.
Patent Literature 3 discloses a method for producing a martensitic stainless steel pipe, in which a stainless steel pipe containing 0.1 to 0.3% of C and 11 to 15% of Cr is quenched by performing a two-stage cooling to obtain a microstructure of which not less than 80% is martensite, and thereafter the stainless steel pipe is tempered, where the two-stage cooling consists of: a first cooling in which air cooling is performed from a quenching onset temperature until when the outer surface temperature becomes any temperature lower than “Ms point—30° C.” and higher than “an intermediate temperature between Ms point and Mf point”; and thereafter a second cooling in which rapid controlled cooling of the pipe outer surface is performed through a temperature range until the outer surface temperature becomes Mf point or lower, so as to ensure an average cooling rate of the pipe inner surface to be not less than 8° C./sec.
The method described in Patent Literature 3 is a method to prevent quench cracking by relatively reducing the cooling rate in the first cooling, and to suppress the formation of retained austenite by the rapid controlled cooling of the pipe outer surface in the second cooling. However, when the wall thickness is heavy, it is difficult to control the cooling rate of the pipe inner surface by cooling the outer surface.
Moreover, Patent Literature 4 discloses, as a method for producing a seamless steel pipe of low alloy steel containing a medium or high level of carbon of C: 0.30 to 0.60%, a method for performing water cooling down to a temperature range of 400 to 600° C. immediately after hot rolling, and after the end of water cooling, performing isothermal transformation heat treatment (austemper process) in a furnace heated to 400 to 600° C. However, the microstructure of the steel pipe which is produced by the isothermal transformation heat treatment according to Patent Literature 4 is bainite which generally has lower strength than martensite, and therefore it may not be able to cope with a case where a high strength is required.